1. Field of the Invention
The present invention generally relates to a viewfinder with a high magnification ratio. In particular, the present invention relates to a variable magnification viewfinder for a lens-shutter camera or an electronic still camera.
2. Description of Related Art
In a Keplarian viewfinder composed of an objective lens having positive refractive power and an eyepiece lens having positive refractive power, a field frame or a reticle is disposed in the vicinity of a focal point of the objective lens. As a result, a region to be photographed or indications of various types of information can be viewed clearly. Thus, the Keplarian viewfinder is especially applicable as a viewfinder in a high-class lens-shutter camera.
Moreover, in the Keplarian viewfinder, an entrance pupil thereof is disposed inside of the viewfinder or on the object-side of the viewfinder. Therefore, there is an advantage in that a diameter of the objective lens does not become very large even when it is utilized in a so-called zoom viewfinder in which magnification of the viewfinder can be continuously varied and when the viewfinder is made to have a wider angle. Accordingly, the Keplarian viewfinder is often particularly used as a zoom viewfinder.
A Keplarian viewfinder including an objective lens having a first lens group with negative refractive power, a second lens group with positive refractive power, and a third lens group with negative refractive power has been used widely. This is because this type of viewfinder has high performance in spite of a compact and simple structure. This type of viewfinder has been disclosed, for example, in Japanese Laid-Open Patent Publication Nos. 3-233420 and No. 6-242377.
In a conventional viewfinder having a high magnification ratio, a field frame is disposed in the vicinity of a real image of an object which is formed via the objective lens, thereby providing a viewer with a clear field at an eyepoint. Therefore, a viewfinder having a high magnification ratio has been widely used in optical products, such as for example, a zoom viewfinder in a compact camera.
However, in a conventional viewfinder having a high magnification ratio, the magnification ratio is, at a maximum, about three. Therefore, the conventional viewfinder cannot go along with recent requirements for increased magnification ratios in the compact cameras.